1. Field of the Invention
This invention relates to a high voltage integrated driver circuit for driving the respective bit lines of a digital computer memory array of floating gate avalanche-injection transistor cells to cause the latter to undergo avalanche breakdown so as to change the floating gate of the selected cell and thereby store on bit of information in the latter. Driver circuits in accordance with the present invention may also be utilized in other applications where a high voltage output swing is required. The present invention also relates to an overall memory array comprising high voltage drivers for the bit lines and the word lines in an array of memory cells each requiring a high voltage for writing information and a low voltage for reading information. Such memory arrays have been commonly referred to as read mostly memories.
2. Description of the Prior Arts
In the prior art of digital computer memories, there has recently been developed a memory cell comprising a transistor with a floating gate charged by avalanche injection. This type of memory cell is called a "floating-gate avalanche-injection metal oxide semiconductor", otherwise known as a "FAMOS" device. This memory cell as well as a word line drive circuit therefore is disclosed in the aforementioned cross referenced U.S. Pat. applicaton by Hansen et al, and is also disclosed in a paper by D. Frohmann-Bentchkowsky entitled, "A Fully-Decoded 2048-Bit Electrically-Programmable MOS ROM", 1971, ISSCC International Solid State Circuits Conference, Feb. 18, 1971.
This memory cell is electrically programmed by applying a high voltage to the respective word line and bit line to cause a PN junction to breakdown so that charge carriers flow to the floating gate and thereby charge the latter. The cell may thereby store a bit of information whose binary value is indicated by the presence or absence of charge on the floating gate. In order to cause the PN junction to undergo avalanche breakdown, it is necessary to drive the word line with a voltage swing which is relatively large compared to the voltages normally utilized in integrated circuits.
The word line driver circuit described in the aforementioned cross referenced patent application by the inventors of the present invention utilizes a combination of bipolar and field effect transistor technology also known as BIFET technology. The processing of integrated circuits including both bipolar and field effect transistors is inherently more complex than the processing of semiconductor chips containing only field effect transistors (FETs). For this reason, a need developed for high voltage driver circuits including only FETs. Moreover, a need developed for such a circuit for driving the bit line during a write cycle and for the same circuit to select the bit line for the read cycle.